1. Field of the Invention
The present invention relates in general to a warning system for a motor vehicle, and more particularly to a so-called `engine oil degradation warning system` which automatically gives a visual or audible warning when the engine oil has been degraded to a certain unusable degree.
2. Description of the Prior Art
As is known, engine oil functions not only to lubricate movable metal parts, such as pistons or the like, but also to remove heat from heated parts of the engine. However, usage of the engine oil degrades the nature thereof gradually with passage of time, and finally the engine oil degrades to an unusable degree.
In view of the above, various kinds of oil degradation warning systems have been hitherto proposed and put into practical use. Some are of a type in which, as is disclosed in Japanese Patent First Provisional Publication No. Sho. 57-151447, the oil change time (viz., the time when a warning for need of oil change is issued) is calculated based on the time for which the engine oil has been `practically` used in the engine That is, in such conventional systems, the time for which the engine has practically operated is integrated and when the integrated value of the time exceeds a predetermined value, the oil change warning is issued for letting a person know the need of oil change.
However, as is known, the degradation of the oil advances naturally with passage of time even When the engine is under a stand still. The advance of oil degradation is most severe when, like just after the engine stops, the oil temperature is still high, and when, in cold weather, the oil is contaminated with water. Thus, some of the conventional warning systems of the above-mentioned type have suffered from the drawback that the oil change warning is displayed considerably after the date on which an the oil change is really needed.
This undesirable phenomenon is much more severe when the engine is equipped with a turbocharger. That is, since the turbocharger is heated so much during operation of the engine, the high temperature of the same is kept long even after stopping of the engine operation. This causes the remarkable degradation of the oil. Thus, when high load operations are frequently carried out in the turbocharger-mounted engine during its use, the time for issuing the oil change warning is greatly delayed, inducing lubrication problems of the engine and the turbocharger.